1. Field of the Invention
The present invention relates to a modulating device and method, demodulating device and method, program, and recording medium, and specifically, relates to a modulating device and method, demodulating device and method, program, and recording medium, wherein information is included in carriers, and the information thereof is processed at a receiving side, thereby realizing high image quality and so forth.
2. Description of the Related Art
FIG. 1 is a diagram illustrating the configuration of an example of a modulating device which performs wireless modulation. A baseband signal input to the modulating device is multiplied by an optional carrier wave at a multiplier 11, and is transmitted to another device (demodulating device). The term “carrier wave”, or simply “carrier”, as used here is a signal for transmitting information by a transmission line (cable communication), by radio (wireless communication), or commonly by wave (light or acoustic wave or the like), i.e., a reference signal not subjected to modulation, with communication.
FIG. 2 is a diagram illustrating the configuration of an example of a demodulating device which performs wireless demodulation. The demodulating device multiplies an input signal (modulated signal) by a signal of which the frequency and phase are synchronized with the carrier employed at the time of modulation at a multiplier 21 again, extracts low-frequency components at an LPF (Low-Pass Filter) 22, thereby reproducing a transmitted signal. With modulation, a method for changing the amplitude, frequency, or phase of a carrier is employed, modulating a carrier enables various information to be transmitted.
FIGS. 3A through 3C are diagrams illustrating a modulation method. FIG. 3A illustrates a case where information is carried in amplitude, and illustrates modulation referred to as amplitude modulation (amplitude shift keying). FIG. 3B illustrates a case where information is carried in frequency, and illustrates modulation referred to as frequency modulation (frequency shift keying). FIG. 3C illustrates a case where information is carried in a phase, and illustrates modulation referred to as phase modulation (phase shift keying).
Such modulation methods, like the amplitude shift keying, frequency shift keying, and phase shift keying, are in common use, but in addition to these modulation methods, there is a modulation method wherein the phase shift keying and amplitude shift keying are compounded, such as QAM (Quadrature Amplitude Modulation) and so forth.
Further, in recent years, the multicarrier method such as OFDM (Orthogonal Frequency Division Multiplexing) and so forth have been employed generally as a technique which applies these. OFDM is a particular case of the multicarrier method for dividing information to be transmitted into multiple carriers to transmit these (hereafter, an information transmission channel according to each carrier will be referred to as “subcarrier”).
FIG. 4 is a diagram illustrating the configuration of an OFDM modulating device. The OFDM modulating device shown in FIG. 4 is configured of a serial/parallel conversion unit 31 (hereafter, described as “S/P conversion unit 31”), IFFT (Inverse Fast Fourier Transform) unit 32, parallel/serial conversion unit 33 (hereafter, described as “P/S conversion unit 33”), guard interval adding unit 34 (hereafter, described as “GI adding unit 34”), D/A (Digital/Analog) conversion units 35-1 and 35-2, low-pass filters (LPFs) 36-1 and 36-2, multipliers 37-1 and 37-2, oscillator 38, adder 39, band-pass filter (BPF) 40, and antenna 41.
The S/P conversion unit 31 converts the data (transmitted data) of each packet (block) which has been encoded, for example, by an unshown encoding unit upstream, and supplied serially, into parallel data for each packet (block), and outputs this to the IFFT unit 32.
The IFFT unit 32 subjects the supplied data to inverse fast Fourier transform (IFFT). The P/S conversion unit 33 converts the data supplied from the IFFT unit 32 in parallel into serial data. A component in phase (I: In phase) and a quadrature component (Q: Quadrature) are each output from the P/S conversion unit 33. The GI adding unit 34 adds a guard interval to data for each symbol supplied from the P/S conversion unit 33.
The D/A conversion unit 35-1 subjects the data of the component in phase output from the GI adding unit 34 to D/A conversion, and the D/A conversion unit 35-2 subjects the data of the quadrature component output from the GI adding unit 34 to D/A conversion. The LPF 36-1 extracts the low-frequency component of the data of the component in phase supplied from the D/A conversion unit 35-1, thereby performing band limiting so as not to cause intersymbol interference. Similarly, the LPF 36-2 extracts only the low-frequency component of the data of the quadrature component supplied from the D/A conversion unit 35-2, thereby performing band limiting so as not to cause intersymbol interference.
The multiplier 37-1 multiplies the data of the component in phase output from the LPF 36-1 by the carrier output from the oscillator 38 (frequency shift keying). Similarly, the multiplier 37-2 multiplies the data of the quadrature component output from the LPF 36-2 by the carrier output from the oscillator 38 (frequency shift keying). The adder 39 adds the data of the component in phase from the multiplier 37-1, and the data of the quadrature component from the multiplier 37-2.
The BPF 40 extracts only the component of a predetermined frequency band (carrier component) of the data supplied from the adder 39, thereby performing band limiting. The output of the BPF 40 is transmitted to the transmission path through the antenna 41.
Description will be made regarding a demodulating device which demodulates the signal thus transmitted. FIG. 5 is a diagram illustrating the configuration of the OFDM demodulating device. The OFDM demodulating device shown in FIG. 5 is configured of an antenna 51, BPF 52, multipliers 53-1 and 53-2, oscillator 54, LPFs 55-1 and 55-2, A/D (Analog/Digital) conversion units 56-1 and 56-2, valid symbol cycle extraction unit 57, S/P conversion unit 58, FFT (Fast Fourier Transform) unit 59, and P/S conversion unit 60.
The demodulating device receives the transmitted data through the antenna 51. The BPF 52 removes unnecessary band components from the data received through the antenna 51 to extract only a carrier component. The multipliers 53-1 and 53-2 each multiply the output of the BPF 52 by the carrier component oscillated from the oscillator 54, thereby performing frequency conversion processing.
The LPF 55-1 extracts only the data including a subcarrier component (baseband component) from the data output from the multiplier 53-1, and outputs this to the A/D conversion unit 56-1. Similarly, the LPF 55-2 extracts only the data including a subcarrier component (baseband component) from the data output from the multiplier 53-2, and outputs this to the A/D conversion unit 56-2.
The A/D conversion units 56-1 and 56-2 each subject the input data to A/D conversion. The valid symbol cycle extraction unit 57 extracts only a valid symbol portion from the data supplied from each of the A/D conversion unit 56-1 and 56-2. The S/P conversion unit 58 converts data supplied in serial into data in parallel, and outputs this to the FFT unit 59.
The FFT unit 59 subjects the valid symbol portion supplied from the valid symbol cycle extraction unit 57 to FFT processing to output this to the P/S conversion unit 60. The P/S conversion unit 60 subjects the data supplied from the FFT unit 59 to serial/parallel conversion. Thus, the modulated data is demodulated.
The following points can be listed as the features of the OFDM method. First, as the advantages of OFDM, the following can be listed.    1. A great number of subcarriers are employed, and accordingly can handle frequency-selective phasing at multi-path transmission paths well.    2. Not only time interleaving but also frequency interleaving can be performed, thereby using error correction effects effectively.    3. The symbol cycle is long, and further a GI (guard interval) is provided, whereby disturbance due to reflected waves can be reduced.    4. Each carrier of OFDM is a digital modulation wave having a low bit rate and a narrow band, whereby the spectrum of each subchannel can be disposed tightly, and accordingly, frequency use efficiency is high.    5. Flexible information transmission can be performed such that a channel from which interference is expected is not employed.    6. Hierarchization of information can be readily performed by changing the modulation method of each subcarrier, or the like.
Of the above-mentioned advantages, a method has been proposed in J. Fu and Y. Karasawa, “Fundamental Analysis on Throughput Characteristics of Orthogonal Frequency Division Multiple Access (OFDMA) in Multipath Propagation Environments”, IEICE journal, vol. J85-B, no. 11, pp. 1884-1894, November 2002, wherein “flexible information transmission can be performed such that a channel from which interference is expected is not employed” cited as the above-mentioned advantage 5 is utilized, and a subcarrier of which the influence of phasing is weak is selected and assigned to a user. According to this method, though complexity of the system increases, a subcarrier having small influence of phasing is selected, whereby satisfactory BER (Bit Error Ratio) characteristics can be obtained.